[EAS] The Dark Side of the Universe

Peter J. Kindlmann peter.kindlmann at yale.edu
Thu Feb 23 00:01:00 EST 2012

Dear Friends and Colleagues -


Another example of fine science writing in The 
Economist, this issue's Science and Technology 
section is devoted to cosmology and the puzzle 
why the universe's expansion isn't slowing down 
but speeding up.

Remarkably for a publication whose title would 
not suggest it, The Economist has a long history 
of fine science writing, much of it due to that 
section's late editor, Richard Casement. In a 
remarkable instance so long ago that it seems 
beyond archival searching, he devoted an entire 
Science and Technology section to the (then) 
current state of quantum electrodynamics, 
accessible to the layman.



The dark side of the universe
Scientists are trying to understand why the universe is running away from them

Feb 18th 2012 | from the print edition

AT FIVE tonnes and 520 megapixels, it is the 
biggest digital camera ever built-which is 
fitting, because it is designed to tackle the 
biggest problem in the universe. On February 20th 
researchers at the Cerro Tololo Inter-American 
Observatory (pictured), which sits 2,200 metres 
(7,200 feet) above sea level in the Atacama 
desert of northern Chile, will begin installing 
this behemoth on a telescope called Blanco. It is 
the centrepiece of the Dark Energy Survey (DES), 
the most ambitious attempt yet to understand a 
mystery as perplexing as any that faces physics: 
what is driving the universe to expand at an ever 
greater rate.

It has been known since the late 1920s that the 
universe is getting bigger. But it was thought 
that the expansion was slowing. When in 1998 two 
independent studies reached the opposite 
conclusion, cosmology was knocked head over 
heels. Since then, 5,000 papers have been written 
to try to explain (or explain away) this result. 
"That's more than one a day," marvels Saul 
Perlmutter, of the Lawrence Berkeley National 
Laboratory, who led the Supernova Cosmology 
Project-one of the studies that was responsible 
for dropping the bombshell. Last October that 
work earned Dr Perlmutter the Nobel prize for 
physics, which he shared with Brian Schmidt and 
Adam Riess, who led the other study, the High-Z 
Supernova Search.

Many of those 5,000 papers deal with something 
that has come to be known as dark energy. One 
reason for its popularity is that, at one fell 
swoop, it explains another big cosmological find 
of recent years. In the early 1990s studies of 
the cosmic microwave background (CMB), an 
all-pervading sea of microwaves which reveals 
what the universe looked like when it was just 
380,000 years old, showed that the universe, then 
and now, was "flat". However big a triangle you 
draw on it-the corners could be billions of light 
years apart-the angles in it would add up to 
180°, just as they do in a school exercise book.

That might not surprise people whose geometrical 
endeavours have never gone beyond such books. But 
it surprised many physicists. At some scales 
space is not at all flat: the power of Albert 
Einstein's theory of general relativity lies in 
its interpretation of gravity in terms of curved 
space. Cosmologists were quite prepared for it to 
be curved at the grandest of scales, and 
intrigued to discover that it was not.

Dark thoughts

Relativity says that for the universe to be flat, 
it has to have a very particular density-which in 
relativity is a measure not just of the mass 
contained in a certain volume, but also of the 
energy. The puzzle was that various lines of 
evidence showed that the universe's endowment of 
ordinary matter (the stuff that people, planets 
and stars are made of) would give it just 4% of 
that density. Adding in extraordinary 
matter-"dark matter", not made of atoms, that 
interacts with the rest of the universe almost 
only by means of gravity-gets at most an extra 
22%. That left almost three-quarters of the 
critical density unaccounted for. Theorists such 
as Michael Turner, of the University of Chicago, 
became convinced that there was something big 
missing from their picture of the universe.

Whatever it is that is driving the universe's 
accelerating expansion fits the bill rather well. 
Add the amount of energy needed to keep cosmic 
acceleration going to the amount of matter and 
energy in the universe already accounted for and 
you have more or less exactly the density of 
matter and energy needed to make the universe 
flat. But there is a catch; for the sums to 
tally, that "dark energy"-Dr Turner is thought to 
have coined the term- must be very strange stuff 
indeed. According to Einstein's theory of 
relativity, energy in the form of radiation has 
the same sort of gravitational effect as matter 
does-the photons of which light is made exert a 
pressure, and this in turn gives rise to a 
gravitational attraction. In order to drive its 
acceleration, then, dark energy must instead have 
a repulsive effect. It must, in other words, 
exert a negative pressure.

Divide dark energy's pressure (negative) by its 
energy density (positive) and you get something 
cosmologists label "w". It is easy to see that w 
must be negative. Observations made since 1998 
suggest that w is pretty close to -1. If it were 
found to be exactly -1, that would make dark 
energy something physicists call a cosmological 
constant. A cosmological constant is the same no 
matter where in the universe you look-an 
inherent, unchanging feature of the fabric of 
creation, however much it expands, twists or ties 
itself in knots.

The cosmological constant is another thing first 
dreamed up by Einstein. On realising that the 
equations of general relativity allowed for the 
universe's expansion (or, indeed, contraction), 
he added a parameter describing just such a 
constant in order to keep it from doing either. 
For all his notoriously counterintuitive 
predictions, an expanding universe was one he was 
not prepared to countenance, at least not in 
1917, when he published his theory. After Edwin 
Hubble's discovery 12 years later that other 
galaxies were indeed streaming away from Earth's 
Milky Way backyard, Einstein dropped the tweak. 
No doubt miffed that he had not trusted his maths 
in the first place, he later called the 
cosmological constant his "biggest blunder".

By then, though, the cosmological constant had 
been seized upon by quantum theorists, themselves 
in the midst of turning physics on its head. 
Quantum theory says that the seemingly empty 
vacuum of space is, in fact, not empty at all. 
Instead it is constantly abuzz with "virtual" 
particles flitting in and out of existence. The 
energy resulting from all this buzzing-vacuum 
energy-should be a fixed feature of space-in 
other words, a cosmological constant.

Stringing it all together

And, in principle, it could also propel the 
universe's expansion. Thus vacuum energy and dark 
energy might be the same thing. But this 
theoretical neatness runs into a practical 
problem. A naive approach to quantum theory says 
that vacuum energy should be a whopping 1060 to 
10120 times bigger than dark energy's estimated 
energy density. Some physicists call this "the 
worst prediction ever". Working out why vacuum 
energy is not so vast has been a problem for 
physics ever since.

Cliff Burgess, from Perimeter Institute for 
Theoretical Physics in Waterloo, Ontario, and the 
author of a handful of the 5,000 papers Dr 
Perlmutter has dug up, thinks he has a solution; 
the vacuum energy is vast, but it is almost all 
hidden away in extra spatial dimensions. Unlike 
the familiar three of length, breadth and height, 
these extra dimensions are curled up so tightly 
that they elude detection (though scientists are 
trying to prise them open in particle 
accelerators like the Large Hadron Collider near 
Geneva). Extra dimensions are of interest because 
string theory, a class of mathematical models 
based on quantum theory that seeks to describe 
reality in the most fundamental way, requires 
that there be at least six of them, maybe more.

What makes Dr Burgess's proposal unusual is that 
he went out on a limb and suggested that these 
energy-sapping, curled-up extra dimensions should 
be as big as a few microns across, gargantuan by 
string-theory standards. The reason they have not 
been noticed by chipmakers, virologists and 
others who pay attention to things on the micron 
scale, he contends, is that, like dark matter, 
they are sensitive only to gravity, and 
relatively oblivious to the other three of 
nature's fundamental interactions: 
electromagnetism and the weak and strong nuclear 
forces. This may sound like a cheap excuse but it 
makes robust mathematical sense. And it makes 
predictions; at micron scales the attraction 
between two masses will no longer depend on the 
square of the distance between them in the way 
that physicists since Newton have required it to.

An experiment under way at the University of 
Washington, led by Eric Adelberger, tests this 
idea using the world's most sensitive torsion 
balance, a souped-up version of the kit Henry 
Cavendish, an English physicist, used to measure 
gravity directly for the first time in the late 
18th century. It consists of a disk with holes 
around its edge hanging horizontally from a cord, 
microns above another, similarly perforated 
plate. When the bottom disk is rotated the 
material between its holes exerts a tiny 
gravitational tug on the material between the 
holes of the top disk, causing it to rotate, 
albeit only by billionths of a degree. So far, 
Sir Isaac is winning. Dr Adelberger has confirmed 
that Newton's predictions are correct down to 44 
microns. But the experiment continues, and Dr 
Burgess is taking bets that Newton's winning 
streak will not last much longer.

If Dr Burgess is right, vacuum energy and dark 
energy are the same thing, a cosmological 
constant, and w is exactly equal to -1. What, 
though, if it is not? Then dark energy would have 
to be something that varies in space, time, or 
both, and is close to -1 today just by 
coincidence. Names applied to this something else 
include quintessence, k-essence, phantom energy 
and a bunch more, depending on which theorist you 
ask and what properties you think likely. It 
would be a new fundamental force, one that rears 
its head only at vast cosmic distances.

An alternative is to monkey with one of the 
existing forces. Some physicists would rather 
fiddle with Einstein's theory of relativity, for 
instance by making gravity weaker at extremely 
long ranges. This is tricky. It is notoriously 
hard to modify the equations of general 
relativity without damaging the theory beyond 
repair. That is one reason for their enduring 
appeal. Another is that they have been confirmed 
time and again by tests that range from minute 
measurements of bodies circling the solar system 
to observations of the farthest known light 
sources, quasars, billions of light years from 
Earth. Any new theory, then, has its work cut 
out-which has not, of course, stopped theorists 

The more precisely w comes to look like -1, the 
more enthusiasm there will be for cosmological 
constant theories, which require that value, and 
the less enthusiasm there will be for fifth 
forces and modified gravity, part of the charm of 
which is that they can work with other values. 
This is where telescopes like Cerro Tololo come 
in. Existing data from ground-based and space 
telescopes put w at between -1.1 and -0.9. DES 
will aim to narrow the margin of uncertainty down 
to just 0.01. To do so, it will take 400 
one-gigabyte snaps a night for 525 nights over 
five years (the remaining telescope time will be 
split between other science projects). And it 
will use an array of clever techniques to analyse 
the data.

Through a cosmic lens, crookedly

The first is a time-honoured method borrowed from 
Dr Perlmutter, Dr Schmidt and Dr Riess and used 
to study exploding stars called supernovae. These 
come in different varieties. Some, called type 
Ia, always explode with almost exactly the same 
energy. They are, therefore, equally bright. 
Since brightness decreases in a predictable way 
with distance, type Ia supernovae make excellent 
cosmic yardsticks. Since the speed of light is 
constant, knowing how far away such a "standard 
candle" is (calculated from its apparent 
brightness seen from Earth) is to know how long 
ago it exploded. The rate at which stars and 
galaxies are moving away from Earth, meanwhile, 
can be worked out from their redshift. As light 
travels across space, which is stretching, its 
wavelength, too, is stretched and its frequency 
shifts towards the red end of the spectrum. The 
faster the expansion, the greater the redshift.

What the Supernova Cosmology Project and the 
High-z Supernova Search both found, and what 
others have later confirmed, is that distant 
exploding stars are dimmer, and so farther away, 
than their redshift implies they should be if the 
universe has been expanding at a steady clip 
throughout. The expansion must therefore have 
sped up recently.

The two groups originally based this conclusion 
on data from a mere 50-odd supernovae. The number 
has since grown tenfold, but it still leaves 
plenty of wriggle room for the cosmological 
constant to prove, well, not so constant after 
all. Joshua Frieman, who heads DES, hopes his 
team will eventually analyse over 4,000 exploding 
stars, some as far away as 7 billion light years. 
They exploded when the universe was half its 
current age and, researchers now reckon, still 
dominated by the gravity of the matter it 
contained, which was putting the brakes on 
expansion. Dark energy, it is thought, revved 
things up some 5 billion years ago. A better 
estimate of the time at which one gave way to the 
other helps determine w.

Music of the spheres

In addition to supernova searches, which will 
train the telescope at ten patches of the sky 
where Dr Frieman and his colleagues hope to spot 
and track the explosions, DES will be scouring 
one-eighth of the night sky for other clues, 
using three other methods. These all rely on 
throwing cartloads of computing power at 
seemingly random data in order to tease out tiny 
statistical anomalies.

One method looks for the effects of sound waves 
which originated in the Big Bang: baryon-acoustic 
oscillations (BAO). In the Big Bang's primordial 
soup of particles, known as a baryon-photon 
fluid, there were density waves like the sound 
waves in air, though far vaster. When the fluid 
cooled down enough, though, the baryons 
(particles from which atomic nuclei are made) and 
photons parted company. The photons became what 
is now the CMB; it is the fact that they have had 
nothing to do with matter since the Big Bang that 
makes the CMB such a remarkable window into the 
early universe.

With the photons no longer willing to play, there 
could be no more baryon-photon fluid. The baryons 
were stuck in position. Where the oscillations in 
the fluid had bunched the baryons tightly, they 
remained bunched; where they had been rarefied 
they remained sparse. The higher density regions 
became the seeds of galaxies-and the average 
separation of those galaxies thus reveals the 
wavelength of the oscillations in the primordial 
fluid. That characteristic scale has been 
stretched out to around 450m light years; 
measuring it at earlier times is another way to 
show how quickly the universe has been expanding.

The last two of DES's techniques measure not just 
rates of expansion, as supernovae and BAO 
searches do, but also the growth of cosmic 
structures like clusters of galaxies. Tracking 
the size and shape of clusters through time gives 
an idea of the tug-of-war between gravity, 
pulling them together, and dark energy, pushing 
them apart. This could help answer the question 
whether expansion is down to dark energy alone, 
in which case physicists expect a correlation 
between results from all four techniques, or to 
modified gravity, if the last two do not square 
with the first two.

One way to probe structure is to count the number 
of clusters of a given mass in a given volume of 
space at different redshifts. This is harder than 
it sounds because 85% of the mass is invisible 
dark matter. But it can be measured indirectly, 
for instance by looking at how hot clouds of gas 
get as they are pulled towards the cluster's 
dark-matter core by its gravity.

Alternatively, the distribution of matter, both 
dark and humdrum, can be gleaned from the effect 
it has on light. Relativity requires the path of 
light to be bent by massive objects. The heavier 
the object, the more an image of something behind 
it is warped. Most of the time, this warping is 
tiny-images of galaxies are typically stretched 
by 2% or so by the clumps of matter they pass on 
their way to telescopes on Earth. To complicate 
matters further, few galaxies are perfectly round 
to start with, so it is hard to tell whether 
stretching has taken place by looking at any 
particular galaxy. Fortunately, light from all 
the galaxies in a given region of the sky passes 
by the same clumps of matter on the way to Earth. 
So galaxies as seen from Earth ought all to be 
distorted in a preferred direction. Observe 
enough of them, 300m in DES's case, and a pattern 
should emerge, allowing astronomers to model the 
structures responsible for the bending.

Combine all four techniques and a clearer picture 
of the causes of cosmic acceleration will emerge. 
That, at least, is the hope. Ofer Lahav from 
University College, London, who is in charge of 
DES's science programme, says the odds are that 
DES will home in on w being equal to -1-some sort 
of a cosmological constant.

Saving the best 'til LSST

Other, even more ambitious projects, will strive 
to increase the precision of the measurement of 
W. Last year ground was broken on the Large 
Synoptic Survey Telescope (LSST), a much bigger 
instrument which will be perched atop Cerro 
Pachón, 10km (6 miles) from Cerro Tololo. Though 
its $620m budget awaits final approval from 
America's National Science Foundation and 
Department of Energy, scientists hope to have it 
up and running by 2021. The LSST's mammoth camera 
will boast 3.2 gigapixels.

Then there are two space telescopes, each with a 
price tag of $1 billion or so. The European Space 
Agency plans to launch Euclid in 2019 and NASA 
hopes to put WFIRST in orbit three years later.

These projects are not solely dedicated to 
probing the nature of dark energy. LSST, for 
example, will discover asteroids by the 
bushel-including some that might be hazardous to 
Earth. But one way or another it is cosmic 
expansion that they, and all sorts of other 
astronomical ventures, will be addressing.

The rub is that no amount of observations can 
ever pin down the figure for w with perfect 
accuracy. That would require infinite precision, 
something impossible to achieve even in an 
ever-expanding universe. And the whole constant 
idea falls to pieces if w is even a smidgen off 

More than any other scientific problem the 
cosmic-expansion conundrum presents scientists 
with an existential quandary. "It could be a 
22nd-century problem we stumbled upon in the 20th 
century," says Dr Turner. Some researchers may 
begin to feel time would be better spent on other 
scientific pursuits.

Many astronomers, including Dr Perlmutter, are 
quietly hoping that as DES and the host of other 
acronyms come online, they will spring another 
surprise, like the one that first propelled 
cosmic acceleration into the limelight in 1998. 
Whether they do or not, though, dark energy-or 
whatever else is causing the universe to speed 
up-is probably too big a conundrum for one 
generation to crack. It will cause boffins to 
rack their brains for years to come.

from the print edition | Science and technology

More information about the EAS-INFO mailing list